Since this morning we can't research by prefix anymore, no matter which prefix. They were working before.
To reproduce the bug, just click on https://fr.wiktionary.org/wiki/Sp%C3%A9cial:Recherche?search=%22flexions+de+verbes%22&prefix=Wiktionnaire%3AWikid%C3%A9mie&fulltext=Rechercher+dans+les+archives+de+la+Wikid%C3%A9mie&fulltext=Search&searchToken=a5gj3iqakrsztpaixpiynhxv0
The first bug witnesses are discussing here: https://fr.wiktionary.org/wiki/Wiktionnaire:Questions_techniques/mai_2018